An operation of the above kind of a laser diode is disclosed in the paper of H. Shimade et al entitled "Measurement of Coherence Characteristics of Semiconductor Laser Driven by High-Frequency Current Injection", The Transactions of the IEICE, volume E70, No. 11, November 1987. In this operation of a laser diode, a high-frequency current having a sinusoidal waveform is superposed on the injection current in order to reduce the coherence length of the emitted radiation. A reduction of the coherence length is advantageous in numerous devices which are based on interference because the signal-to-noise ratio is improved in this manner. In the above-mentioned paper, the current is modulated which directly effects a modulation of the wavelength or wave number of the emitted radiation and thereby changes the coherence length of the radiation. Since the wavelength or wave number can also be changed by the operating temperature of the laser diode, it is more appropriate to speak of the modulation of the wave number because the spectral distribution of the emitted radiation directly influences the coherence function.
For the signal-to-noise ratio of specific interferometric arrangements, not only is the coherence length of the radiation used decisive, but the overall coherence function, that is, the coherence coefficient as a function of the optical path difference. Most advantageous is a coherence function having a high coherence coefficient within the desired coherence length and a steepest possible and above all a continuous drop outside of the coherence length. Secondary maxima of the coherence coefficient outside of the coherence length are especially undesirable.
A disadvantage of the sinusoidal modulation of the injection current known from the above-mentioned paper is that for single-mode as well as multi-mode laser diodes numerous secondary maxima occur outside of the desired coherence length. For multi-mode diodes, the secondary maxima occur for optical path differences of a multiple of the double optical resonator length. For single-mode diodes, the position of the secondary maxima is primarily dependent upon the modulation amplitude.